What primary mechanism causes reflection cracking in asphalt overlays?

Reflection cracking in asphalt overlays is primarily caused by stress concentrations above existing cracks or joints in the underlying pavement layer. These stress concentrations occur because the overlay is thinner and less stiff than the original pavement. The existing cracks or joints create areas where the pavement is weaker and more likely to deform under traffic loads. As vehicles repeatedly pass over these areas, the stresses from the load are focused directly above the existing discontinuity. This concentration of tensile stress exceeds the tensile strength of the asphalt overlay, causing a new crack to initiate at the surface of the overlay directly above the existing crack or joint. The crack then propagates through the overlay, eventually becoming visible as reflection cracking. Factors such as overlay thickness, asphalt mix design, traffic volume, and temperature variations can influence the rate at which reflection cracking develops, but the underlying mechanism is the stress concentration due to the pre-existing cracks or joints.